Method of determining motion vectors and a reference picture index for a current block in a picture to be decoded

ABSTRACT

In one embodiment, the method includes obtaining first and second motion vectors of blocks other than the current block. The other blocks neighbor the current block at one of a left, top and top right position. A direction of the first and second motion vectors of the at least one neighboring block is obtained by a decoder. First and second prediction motion vectors of the current block are determined based on the obtained directions of the at least one neighboring block and by applying a median operation to the first motion vectors of the at least one neighboring block and applying a median operation to the second motion vectors of the at least one neighboring block.

DOMESTIC PRIORITY INFORMATION

This is a divisional of, and claims priority under 35 U.S.C. §120 to,U.S. application Ser. No. 10/337,808, filed Jan. 6, 2003, herebyincorporated by reference in its entirety

FOREIGN PRIORITY INFORMATION

The present invention claims priority under 35 U.S.C. 119 on KoreanApplication Nos. 2002-42204 and 2002-44162 which were filed on Jul. 18,2002 and Jul. 26, 2002, respectively; the contents of all of which arehereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a moving picture coding system, andmore particularly to a prediction motion vector calculation method bydefining a motion vector to be used in a median operation when aneighboring block around a block to be coded has a plurality of motionvectors, so as to obtain a prediction motion vector (PMV) of the blockto be coded; using motion vector information of neighboring blocks, andimprove a coding efficiency.

2. Description of the Related Art

Generally, in order to reduce the amount of bits to be used for thetransfer of motion information, an encoder, instead of sending a motionvector MV directly to a decoder, selects a median value of motionvectors of three neighboring blocks through a median operation,determines the selected median value as a prediction motion vector PMV,obtains a difference MVD between the MV and the PMV (i.e., MVD=MV−PMV),and sends the obtained difference MVD to the decoder.

Then, the decoder obtains the motion vector MV by obtaining theprediction motion vector PMV in the same manner as the encoder andadding the sent MVD to the obtained PMV. In FIG. 1, a block E is a blockto be coded (or decoded), and blocks A, B and C are neighboring blocksof the block E. Defining motion vectors of the neighboring blocks A, Band C, respectively, as MV_(A), MV_(B) and MV_(C), a prediction motionvector PMV of the block E can be obtained through a median operation asfollows:PMV=median {MV_(A),MV_(B),MV_(C)}

A block D in FIG. 1 is a block which is used instead of the block C whenthe block C exists outside of a picture. Provided that only one of thethree blocks A, B and C, or A, B and D refers to the same referencepicture as that referred to by the block E, a motion vector MV of thatblock will be used as the prediction motion vector PMV. This motioninformation sending method is, applied to all pictures irrespective oftheir types.

On the other hand, a B picture has five types of predictive modes suchas forward mode, backward mode, bi-predictive mode, direct mode andintra mode. Generally, a neighboring block in the forward mode has onemotion vector MVFW obtained from a forward reference picture with anindex ref_idx_fwd, and a neighboring block in the backward mode has onemotion vector MVBW obtained from a backward reference picture with anindex ref_idx_bwd.

In the bi-predictive mode of the B picture, the prediction is allowedfrom different directions and the same directions, such asforward/forward, backward/backward, and forward/backward. Each referencepicture uses the index ref_idx_fwd or ref_idx_bwd regardless of itsdirection (forward or backward), and each motion vector is alsorepresented as MVFW or MVBW regardless of its direction (The reason isthat the predefined ‘syntaxes’ are used as they are. For expression ofthe syntaxes, ‘ref_idx_(—)10’ or ‘ref_idx_(—)11’ may be used for eachindex and ‘mv_list0’ or ‘mv_list1’ may be used for each motion vector.).The direct mode of the B picture is a predictive mode where motioninformation is not sent to the decoder and motion vectors MVf and MVband reference pictures are derived from the inside of the decoder. Thefact that the derived motion vectors are represented as MVf and MVbirrespective of their directions is the same as that in thebi-predictive mode.

In a conventional method for calculating a prediction motion vector PMVof the B picture, a forward prediction motion vector of the block E isobtained by extracting only forward motion vectors of the neighboringblocks and performing a median operation with respect to the extractedforward motion vectors. If one of the neighboring blocks has no forwardmotion vector, its motion vector is set to 0 and the median operation isperformed under such a condition. This method is similarly applied to abackward prediction motion vector of the block E, so as to use onlybackward motion vectors of the neighboring blocks. If one of theneighboring blocks is in the intra mode, its motion vector is set to 0,the neighboring block is considered to refer to a reference picturedifferent from that referred to by the block E, and the predictionmotion vector PMV is obtained under such a condition.

However, as stated above, in the bi-predictive mode of the B picture,the prediction is allowed from different directions and the samedirections, such as forward/forward, backward/backward, andforward/backward, each reference picture uses the index ref_idx_fwd orref_idx_bwd regardless of its direction (forward or backward), and eachmotion vector is also represented as MVFW or MVBW regardless of itsdirection. As a result, there is a need to define a method forcalculating a prediction motion vector PMV when a neighboring blockhaving two motion vectors exists.

Provided that a neighboring block is in the bi-predictive mode (or thedirect mode), motion vectors MVFW and MVBW (or MVf and MVb) thereof mayhave the same directions such as forward/forward or backward/backward,or different directions such as forward/backward. This directioninformation of the motion vectors cannot be determined from only themotion vector syntaxes ‘MVFW’ and ‘MVBW’ or the reference pictureindexes ‘ref_idx_fwd’ and ‘ref_idx_bwd’. The conventional method forcalculating the PMV of the B picture gives no accurate description ofsuch a problem, resulting in great confusion.

For example, in the case where a neighboring block is in thebi-predictive mode having two motion vectors in the forward/forwarddirections, the conventional PMV calculation method gives no clearlydefined determination as to whether both or any one of the two motionvectors must be used for the calculation of the forward predictionmotion vector PMV of the block E.

SUMMARY OF THE INVENTION

The present invention relates to a method for assigning directioninformation to reference pictures and a method for determining thedirections of the reference pictures, wherein unique informationenabling the acquisition of direction information of motion vectors isassigned to each reference picture, so that information regarding adirection from each neighboring block to each reference picture can beacquired.

The present invention relates to providing a prediction motion vectorcalculation method by defining a motion vector to be used in a medianoperation when a neighboring block of a block to be coded has aplurality of motion vectors, so as to obtain a prediction motion vector(PMV) of the block to be coded, using motion vector information ofneighboring blocks, and improve a coding efficiency.

In one embodiment, direction information is assigned to a referencepicture as a feature of the reference picture, so as to give thedirection information of the reference picture pointed by referencepicture index.

The direction information may be indicative of the display order of eachreference picture may be represented by a picture order count (POC)value.

In accordance with another aspect of the present invention, there isprovided a method for determining directions of reference picturespointed to, respectively, by reference picture indexes, comprising thestep of acquiring display order information of each reference picture,comparing display order information with display order information of ablock to be currently coded, and determining a direction (forward orbackward) of each reference picture against the block to be currentlycoded.

The display order information of each reference picture may be acquiredfrom a POC value.

In accordance with yet another aspect of the present invention, there isprovided a method for calculating a prediction motion vector (PMV) of ablock to be coded, by performing a median operation using motion vectorsof neighboring blocks, comprising the steps of a), if the neighboringblocks have the motion vectors, acquiring direction information ofreference pictures pointed by the motion vectors of the neighboringblocks; and b) selecting ones of the motion vectors of the neighboringblocks with reference to the acquired direction information andperforming the median operation including the selected motion vectors toobtain the prediction motion vector of the block to be coded.

The step a) may include the step of determining the directioninformation of the motion vectors by comparing display order informationof the reference pictures pointed by the motion vectors of theneighboring blocks with display order information of the block to becoded.

The step b) may include the step of, if one of the neighboring blockshas two motion vectors with different directions, selecting one of thetwo motion vectors having the same direction as that of the predictionmotion vector and performing the median operation including the selectedmotion vector to obtain the prediction motion vector.

Alternatively, the step b) may include the step of, if one of theneighboring blocks has two motion vectors with the same directions,which are different from that of the prediction motion vector, settingthe two motion vectors to 0, considering the neighboring block to referto a reference picture different from that referred to by the block tobe coded, and performing the median operation including the zero motionto obtain the prediction motion vector.

Alternatively, the step b) may include the step of b-1), if one of theneighboring blocks has two motion vectors MV1 and MV2 with the samedirections, which are the same as that of the prediction motion vector,and both the two motion vectors MV1 and MV2 refer to the same referencepicture, selecting one of the two motion vectors MV1 and MV2 andperforming the median operation including the selected motion vector toobtain the prediction motion vector.

The step b-1) may include the step of b-2) selecting one of the twomotion vectors MV1 and MV2 to be earlier decoded or having the same mode(MV1 mode or MV2 mode) as that of the prediction motion vector, andperforming the median operation including the selected motion vector toobtain the prediction motion vector. Here, the motion vector having thesame mode signifies a motion vector having the same transcription asthat indicative of the prediction motion vector.

Alternatively, the step b) may include the step of, if one of theneighboring blocks has two motion vectors MV1 and MV2 with the samedirections, which are the same as that of the prediction motion vector,and only one of the motion vectors MV1 and MV2 refers to a referencepicture referred to by the block to be coded, selecting one of themotion vectors MV1 and MV2 referring to the reference picture referredto by the block to be coded, and performing the median operationincluding the selected motion vector to obtain the prediction motionvector.

Alternatively, the step b) may include the step of, if one of theneighboring blocks has two motion vectors MV1 and MV2 with the samedirections, which are the same as that of the prediction motion vector,neither of the motion vectors MV1 and MV2 refers to a reference picturereferred to by the block to be coded and they refer to differentreference pictures, selecting one of the motion vectors MV1 and MV2referring to a reference picture closest to the reference picturereferred to by the block to be coded, or a reference picture closest toa picture to be currently coded, and performing the median operationincluding the selected motion vector to obtain the prediction motionvector.

Alternatively, the step b) may include the step of, if one of theneighboring blocks has one motion vector with a direction different fromthat of the prediction motion vector, setting the motion vector of theneighboring block to 0, considering the neighboring block to refer to areference picture different from that referred to by the block to becoded, and performing the median operation including the zero motion ofthe neighboring block to obtain the prediction motion vector.

Alternatively, the step b) may include the step of, if one of theneighboring blocks has one motion vector with the same direction as thatof the prediction motion vector, performing the median operationincluding the motion vector of the neighboring block to obtain theprediction motion vector.

The present invention still further relates to a method of determiningmotion vectors and a reference picture index for a current block in apicture to be decoded.

In one embodiment, the method includes obtaining first and second motionvectors of at least one block other than the current block. The otherblocks neighbor the current block at one of a left, top and top rightposition. A direction of the first and second motion vectors of the atleast one neighboring block is obtained based on a comparison of apicture order count value of a current picture and the picture ordercount value of a reference picture referred by the at least oneneighboring block. First and second prediction motion vectors of thecurrent block are determined based on the obtained directions of theleast one neighboring block and by applying a median operation to thefirst motion vectors of the at least one neighboring blocks and applyinga median operation to the second motion vectors of the at least oneneighboring block.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent to those of ordinary skill in the art bydescribing in detail example embodiments thereof with reference to theattached drawings in which:

FIG. 1 is a view illustrating the calculation of a prediction motionvector of a block E using motion vectors of neighboring blocks A, B andC.

FIG. 2 illustrates a process of determining a direction of a motionvector for a current block in a picture to be decoded according to anexample embodiment of the present invention.

FIG. 3 is a flow chart illustrating a motion vector calculation for acase where two motion vectors of a neighboring block have differentdirections according to an example embodiment of the present invention.

FIG. 4 is a flow chart illustrating a motion vector calculation for acase where two motion vectors of a neighboring block have the samedirections, which are the same as that of a prediction motion vector,according to another example embodiment of the present invention.

FIG. 5 is a flow chart illustrating a motion vector calculation for acase where two motion vectors of a neighboring block have the samedirections, which are different from that of a prediction motion vector,according to another example embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

If a neighboring block of a block to be coded is in a bi-predictive mode(or a direct mode), motion vectors MVFW and MVBW (or MVf and MVb)thereof may have the same directions such as forward/forward andbackward/backward, or different directions such as forward/backward.This direction information of the motion vectors cannot be determinedfrom only motion vector syntaxes ‘MVFW’ and ‘MVBW’ or reference pictureindexes ‘ref_idx_fwd’ and ‘ref_idx_bwd’. For this reason, there is aneed to acquire the direction information by referring to differentunique information held by reference pictures.

This invention proposes a method for acquiring direction information ofmotion vectors by comparing the display orders of reference pictures,and calculating a prediction motion vector PMV on the basis of theacquired direction information.

Now, example embodiments of the present invention will be described indetail with reference to the annexed drawing.

1. Recognition of Motion Vector Directions Through. Display OrderComparison

Direction information of motion vectors of neighboring blocks must beacquired before calculation of a prediction motion vector PMV of a blockto be coded. Then, through the direction information of the motionvectors of the respective neighboring blocks, determinations are made asto whether the motion vectors of the neighboring blocks must be includedin the median operation.

In general, if a neighboring block is in a forward mode or backward modehaving one motion vector, the direction of that motion vector can bedetermined from a reference picture index. However, in the case where aneighboring block is in a bi-predictive mode or direct mode having twomotion vectors, it is impossible to recognize actual directions ofreference pictures pointed to by two reference picture indexes.

The reason is as follows. A decoder of a moving picture coding systemwhich allows multiple reference pictures and a B picture to be used asreferences cannot estimate direction information only with referencepicture indexes because it cannot accurately acquire the number offorward reference pictures and backward reference pictures of the Bpicture to be currently decoded, even though it can recognize therelation between a default forward/backward indexing order and arelative forward/backward indexing order from re-mapping informationsent from an encoder.

In the present invention, it is proposed that a reference picturepointed to by a reference picture index will include unique informationindicative of its display order for recognition of its direction. Thisdisplay order is represented by a picture order count (POC) value. FIG.2 illustrates a process of determining a direction of a motion vectorfor a current block in a picture to be decoded according to an exampleembodiment of the present invention. In the example embodiment of FIG.2, the direction of each motion vector can be easily recognized (stepS125) by comparing, in step S115, a display order of each referencepicture pointed to by each reference picture index with a display orderpicture of the B picture to be currently coded, the display orders ofwhich are obtained in step S100. As is well-known in the art, thecomparison may be based on an inequality determination. Accordingly, themotion vector(s) for the current block are predicted based upon theoperations described below with respect to scenarios 2.1, 2.2 or 2.3,based upon the determined direction for each motion vector.

2. Median Operation for Calculation of Prediction Motion Vector PMV WhenNeighboring Block is in Bi-Predictive Mode or Direct Mode

If a neighboring block of a block to be coded is in the bi-predictivemode or direct mode, it has two motion vectors. Of these motion vectors,one having the same direction as that of a prediction motion vector PMVof the block to be coded will have to be used for the median operation.A detailed description will hereinafter be given of the efficientprediction motion vector PMV calculation method proposed by the presentinvention. For the convenience of description, assume that a block to becoded is E, neighboring blocks are A, B, C and D, and two motion vectorsof each neighboring block are MV1 and MV2, as shown in FIG. 1.

2.1 Case where Two Motion Vectors of Neighboring Block Have DifferentDirections

FIG. 3 is a flow chart illustrating a motion vector calculation for acase where two motion vectors of a neighboring block have differentdirections according to an example embodiment of the present invention.

A motion vector having the same direction as that of a prediction motionvector PMV of a block to be coded is selected and the prediction motionvector PMV is then calculated through the median operation.

In other words, referring to the example embodiment of FIG. 3, a forwardmotion vector is selected for calculation of the PMV with respect to aforward prediction motion vector of the block E in step S200, and abackward motion vector is selected for calculation of the PMV withrespect to a backward prediction motion vector of the block E in stepS205. Then, the prediction motion vector PMV of each direction isobtained through the median operation in step S210.

2.2 Case Where Two Motion Vectors of Neighboring Block have the SameDirections, which are the Same as that of Prediction Motion Vector PMV

FIG. 4 is a flow chart illustrating a motion vector calculation for acase where two motion vectors of a neighboring block have the samedirections, which are the same as that of a prediction motion vector,according to another example embodiment of the present invention.

In the example embodiment of FIG. 4, in step S300, a determination ismade as to whether two motion vectors MV1 and MV2 of a neighboring blockrefer to a reference picture referred to by the block E to be coded.

If both the motion vectors MV1 and MV2 refer to the same referencepicture (as determined in step S305), one of them (for example, a motionvector to be earlier decoded, or a motion vector having the same mode(MV1 mode or MV2 mode) as that of a prediction motion vector) isselected in step S315 and included in the median operation forcalculation of the prediction motion vector PMV in step S330. Here, themotion vector having the same mode signifies a motion vector having thesame transcription as that indicative of the prediction motion vector.Also, the reference picture referred to by the motion vectors MV1 andMV2 may be the same as or different from the reference picture referredto by the block E to be coded.

Alternatively, if only one of the motion vectors MV1 and MV2 refers tothe reference picture referred to by the block E (as determined in stepS305), it is included in the median operation for calculation of theprediction motion vector PMV in steps S320 and S330.

If neither of the motion vectors MV1 and MV2 refers to the referencepicture of the block E and they refer to different reference pictures(as determined in step S305), one thereof referring to a referencepicture closest to the reference picture referred to by the block E, ora reference picture closest to a picture to be currently coded isselected in step S325 and included in the median operation forcalculation of the prediction motion vector PMV in step S330.

2.3 Case Where Two Motion Vectors of Neighboring Block Have the sameDirections, Which are Different from That of Prediction Motion VectorPMV

FIG. 5 is a flow chart illustrating a motion vector calculation for acase where two motion vectors of a neighboring block have the samedirections, which are different from that of a prediction motion vector,according to another example embodiment of the present invention.

Two motion vectors MV1 and MV2 of a neighboring block are set to 0, theneighboring block is considered to refer to a reference picturedifferent from the reference picture referred to by the block E, and theprediction motion vector PMV of the block to be coded is obtainedthrough the median operation including the zero motion.

In this manner, when a neighboring block has two motion vectors (asdetermined in step S400), the directions of the motion vectors arerecognized from display orders of associated reference pictures in stepS410 and determinations are made as to whether they are the same as thedirection of the prediction motion vector PMV, thereby making itpossible to obtain a PMV more approximating an MV in step S430. Thisresults in a reduction in the magnitude of a motion vector differenceMVD (=MV-PMV) to be sent to the decoder and, in turn, a reduction in theamount of bits to be sent to the decoder. Accordingly, the entire codingefficiency can be raised.

On the other hand, in the case where a neighboring block has one motionvector (as determined in step S400), direction information of the motionvector is acquired from a display order of an associated referencepicture in step S405. If the direction of the motion vector is not thesame as that of the prediction motion vector (as determined in stepS415), the motion vector is set to 0 in step S420, the neighboring blockis considered to refer to a reference picture different from thereference picture referred to by the block to be coded, and theprediction motion vector is obtained through the median operationincluding zero motion in steps S425 and S430.

Further, in the case where a neighboring block has one motion vector (asdetermined in step S400), direction information of the motion vector isacquired from a display order of an associated reference picture in stepS405. If the direction of the motion vector is the same as that of theprediction motion vector (as determined in step S415), the motion vectoris included in the median operation for calculation of the predictionmotion vector in steps S425 and S430.

As apparent from the above description, the present invention provides amethod for assigning direction information to reference pictures and amethod for determining the directions of the reference pictures, whereindisplay order information enabling the acquisition of directioninformation of motion vectors is assigned to each reference picture.Therefore, information regarding a direction from a block to becurrently coded to each reference picture can be acquired.

Further, the present invention provides a prediction motion vectorcalculation method by defining a motion vector to be used in a medianoperation when a neighboring block around a block to be coded has twomotion vectors due to a bi-predictive mode or direct mode of a Bpicture. As a result, a prediction motion vector (PMV) of the block tobe coded can be predicted using motion vector information of neighboringblocks, and a coding efficiency can be improved.

Although the example embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A method of predicting a motion vector for acurrent block in a current picture by a moving picture decoding device,the method comprising: obtaining, by the moving picture decoding device,at least one motion vector of a block other than the current block,wherein said other block is associated with at least one referencepicture indicated by a reference index for a set of multiple referencepictures; obtaining, by the moving picture decoding device, a firstpicture order count value of the current picture, wherein the firstpicture order count value represents a display order of the currentpicture; obtaining, by the moving picture decoding device, a secondpicture order count value of the at least one reference picture, whereinthe second picture order count value represents a display order of theat least one reference picture; comparing, by the moving picturedecoding device, the first picture order count value of the currentpicture with the second picture order count value of the at least onereference picture; predicting, by the moving picture decoding device,the motion vector for the current block using said at least one motionvector, based on the comparison; and decoding the current block in thecurrent picture using the predicted motion vector.